A study was carried out in the USSR to determine the effectiveness of fenthion (sulfidophos 20) against larvae of Hypodermatidae, in order to overcome certain disadvantages of trichlorphon, principally the need to dress animals twice a year. Application of 5-10 ml of the preparation to the backs of calves depressed acetylcholinesterase activity by 32-38%; this effect passed off after 18-20 days.